1. Field of the Invention
The present invention relates to the field of semiconductor integrated circuit manufacturing, and more specifically, to a method of fabricating and repairing a mask with electron beam-induced chemical etching.
2. Discussion of Related Art
After coating photoresist on a semiconductor wafer, a scanner may be used to expose the photoresist to radiation, such as deep ultraviolet (DUV) light with nominal wavelength of 248 nanometers (nm), 193 nm, or 157 nm. The wafer is sub-divided into contiguous identical fields and a reduction projection system is used to scan light across a mask and onto each field. One or more integrated circuit (IC) chips is fabricated in each field. The mask, which may be transmissive or reflective, determines the pattern to be transferred to the photoresist as a result of the exposure followed by a develop process.
Using a Phase-Shifting Mask (PSM) and Optical Proximity Correction (OPC) with DUV light will allow printing of features with a critical dimension (CD) of 100-180 nm. However, Next Generation Lithography (NGL) is required to print features with even smaller CD. Extreme Ultraviolet (EUV) lithography, a leading candidate for NGL, uses exposure light with a central wavelength in the range of 10-15 nm.
An EUV scanner may have 4 imaging mirrors and a Numerical Aperture (NA) of 0.10 to achieve a CD of 50-70 nm with a depth of focus (DOF) of about 1.00 micrometer (um). Alternatively, an EUV scanner may have 6 imaging mirrors and a NA of 0.25 to print a CD of 20-30 nm with a reduction in DOF to about 0.17 um.
A DUV or EUV mask is inspected for defects during fabrication. Repair of critical defects is performed with a focused ion beam (FIB) tool having a Gallium liquid metal ion source. A clear defect is covered up by depositing Carbon or a metal, followed by trimming with gas-assisted etch (GAE). An opaque defect is repaired with physical ion sputtering or GAE with ion bombardment. The process to remove opaque defects should have adequate etch selectivity to the underlying layer. The underlying layer is quartz in a transmissive mask for DUV or a buffer layer in a reflective mask for EUV.
FIB may damage a mask during the scan to search for defects or during the repair of defects. The repaired portions of the mask may be roughened by sputtering. Organic contamination may be deposited on the surface of the mask. Gallium ions may be implanted into underlying layers. Gallium absorbs strongly at 157 nm and at EUV wavelengths, thus decreasing the transmission in a transmissive mask, such as a 157 nm DUV mask, or decreasing the reflectivity in a reflective mask, such as an EUV mask. Underlying layers of the mask may be further damaged by knock-on of atoms by Gallium.
Damage to a mask becomes more problematic as the CD of the features on the mask shrinks. Lowering the acceleration voltage in the FIB will reduce the penetration range of Gallium ions, but etch selectivity and spatial resolution are compromised. Limiting imaging time and overscan area can reduce damage, but repair may also be adversely affected. Post-repair treatment, such as wet etch of the quartz substrate in a 157 nm DUV mask or the buffer layer in an EUV mask, will remove implanted Gallium ions, but the underlying material may become pitted. If sufficient material is removed, a phase error may also be introduced.
Thus, what is needed is an apparatus for and a method of fabricating and repairing a mask without damage.